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Baseline susceptibility to Bacillus thuringiensis toxins and population genetics of the European corn borer, Ostrinia nubilalis (Hubner) (Lepidoptera: Pyralidae)
Abstract
Susceptibility of European corn borer (ECB) populations from the United States and Italy to Bacillus thuringiensis (Bt) proteins CryIA(b) and CryIA(c) was determined. Variation in Bt susceptibility of ECB as measured by growth inhibition was very similar to the variation in mortality. Interpopulation variation in susceptibility to both proteins was observed. The magnitude of interpopulation differences was $\le$4-fold and comparable to the variability observed among generations within a particular population. Additionally, Bt susceptibility was similar among ECB of different pheromone races, voltine ecotypes, and geographic locations. Two diagnostic concentrations were proposed for CryIA(b): 32 ng AI/cm$\sp2$ and 84 ng AI/cm$\sp2,$ corresponding to the estimated EC$\sb{99}$ and LC$\sb{99},$ respectively. Comparable estimates for CryIA(c) were 2 ng AI/cm$\sp2$ (EC$\sb{99})$ and 7 ng AI/cm$\sp2$ (LC$\sb{99}).$ Results from validation experiments using field-collected ECB populations showed that, for CryLA(b), 84 ng AI/cm$\sp2$ is an appropriate diagnostic concentration to use in initial efforts to monitor Bt resistance in ECB. However, for CryIA(c), adjustments and further validation will be required. Genetic divergence among ECB populations was assessed by PCR-RFLP analysis and direct sequencing of both mitochondrial and nuclear (ITS-1) DNA regions. None of the restriction fragment patterns (mitochondrial or nuclear ITS-1) differentiated geographic populations, pheromone races, or voltine ecotypes of ECB. The lack of variation in the ITS-1 was confirmed by direct sequencing. This genetic similarity of ECB populations despite the wide geographic range, physiological, and behavioral differences may be explained by gene flow and/or an expansion from a genetic bottleneck. The inability to identify polymorphic DNA markers precluded quantification of gene Bow among ECB populations, information necessary to predict the potential for spread of Bt resistance genes. Baseline susceptibility data support the assumption that ECB populations from across the geographic distribution of the species are susceptible to both CryIA(b) and CryIA(c) and provide a basis for monitoring changes in susceptibility resulting from selection with Bt. The diagnostic bioassay developed in this study is an essential tool for effectively monitoring Bt resistance status of ECB field populations as they become exposed to transgenic Bt corn.
Subject Area
Entomology|Genetics|Molecular biology
Recommended Citation
Marcon, Paula Cristina R. Gouveia, "Baseline susceptibility to Bacillus thuringiensis toxins and population genetics of the European corn borer, Ostrinia nubilalis (Hubner) (Lepidoptera: Pyralidae)" (1997). ETD collection for University of Nebraska-Lincoln. AAI9804333.
https://digitalcommons.unl.edu/dissertations/AAI9804333